1. Field of the Invention
The present invention relates to an optical amplifier employing a rare earth element doped fiber or a rare earth element doped optical waveguide doped with a rare earth element.
2. Description of the Related Art
In optical communication systems used at present, a repeater is inserted at fixed distance intervals, so as to compensate the attenuation of an optical signal due to a power loss of an optical fiber. The repeater is constructed in such a manner that the optical signal is converted into an electrical signal by a photodiode followed by amplification of the electrical signal by means of an electronic amplifier. Thereafter the electrical signal thus amplified is converted into an optical signal by means of a semiconductor laser or the like followed by returning the optical signal to an optical transmission line, if the optical signal can be directly amplified with a low noise without conversion into an electrical signal, the optical repeater can be made compact and economized.
In this circumstance, much research has been undertaken to develop an optical amplifier capable of directly amplifying an optical signal. The optical amplifiers which are the subject of this research are generally classified into (a) an optical amplifier employing, in combination, an optical fiber doped with a rare earth element (Er, Nb, Yb, etc.) and a pumping light; (b) an optical amplifier employing a semiconductor laser doped with the rare earth element; and (c) an optical amplifier utilizing a nonlinear effect in the optical fiber.
Above all, the optical amplifier employing the combination of the rare earth element doped fiber and the pumping light, as mentioned in the above type (a), has excellent features, such as no polarization dependency, low noise, and small coupling loss to a transmission line. Accordingly, the optical amplifier of this type is expected to remarkably increase a repeating distance in an optical fiber transmission system, and it is also expected to enable multiple distributions of the optical signal.
FIG. 1 shows the principle of the optical amplification by the rare earth element doped fiber. Referring to FIG. 1, reference numeral 2 designates an optical fiber constructed of a core 2a and a clad 2b. Erbium (Er) is doped in the core 2a. When a pumping light is input into the Er doped fiber 2, Er atoms are excited up to a high energy level. When a signal light is input into the optical fiber 2 having Er atoms excited up to the high energy level, the Er atoms are shifted to a low energy level. At this time, stimulated emission of light is generated, and a power of the signal light is gradually increased along the optical fiber, thus effecting amplification of the signal light.
In general, the concentration of atoms doped in the core 2a is uniform with respect to a longitudinal direction and a radial direction of the Er doped fiber 2.
In accordance with the above-mentioned principle of the optical amplification, when the rare earth atoms in the rare earth element doped fiber are excited up to a high energy level by the pumping light, the energy of the pumping light is consumed. Therefore, as being propagated in the rare earth element doped fiber, the power of the pumping light is absorbed. Meanwhile, it is known that if the power of the pumping light is less than a certain threshold level, there does not occur the excitation of the rare earth atoms enough to effect the optical amplification. Accordingly, in the optical amplifier employing the rare earth element doped fiber doped with the rare earth element at a uniform concentration in the core, the doped rare earth element rather causes a power loss of the signal light and the pumping light. Therefore, the conventional optical amplifier having the above construction is considered to be unsuitable for an increase in amplification efficiency (i.e., a degree of amplification of the signal light with respect to the pumping light having a fixed power).